A radioimmunoassay for multiplication stimulating activity (MSA) has been utilized to show that levels of MSA in fetal rat serum are 20-50-fold higher than in maternal rats. The elevated levels of MSA in fetal rat serum gradually decline to maternal levels by days 15-20 of extrauterine life. A serum somatomedin binding protein has been shown to be growth hormone dependent in man on the basis of a study of growth hormone deficient patients before and after growth hormone treatment.